This invention relates to a method of preparing a sealing glass suspension, also referred to as a paste, for use in a thermal sealing operation wherein a ribbon of suspended sealing glass is applied between parts to be joined, and the glass in the ribbon is thermally softened. It is particularly concerned with a novel method which provides flexibility in storage and transportation practices while maintaining material stability.
Sealing glass suspensions have been used in the fabrication of many different kinds of assemblies comprising ceramic parts, including, for example, glass envelopes for electrical devices, glass-ceramic components for telescope mirror blanks, and ceramic substrates for electrical circuits. Such suspensions are typically provided in paste form, comprising a major proportion of fritted sealing glass and a minor proportion of a pyrolyzable organic vehicle which burns away during the process of firing to effect sealing between the sealing glass and the ceramic parts to be sealed.
One of the most demanding applications for sealing glass suspensions is in the manufacture of cathode ray tube envelopes for television picture tubes and the like. Such envelopes are fabricated by sealing together a glass funnel member and a glass face plate or panel member.
The sealing glass suspension may be applied from a reservoir through an orifice. It is applied as a continuous ribbon or bead to at least one of the mating surfaces provided on the panel and funnel. These parts are then joined and fired to provide an integral assembly wherein the fused sealing glass provides a mechanically strong, high dielectric strength, hermetically sealed joint.
For this and similar applications, the pyrolyzable organic vehicle for the solder glass suspension typically comprises a cellulose binder and an organic solvent for the binder, e.g., a nitrocellulose binder with an amyl acetate solvent. Fritted solder glass additions of about 10-15 parts by weight of fritted glass for each part of organic vehicle are typically used to provide a suspension having the consistency of a paste, suitable for controlled application to a substrate to be sealed.
Sealing glass suspensions have found wide commercial application, and have been the subject of much technical study. However, a problem of shelf life stability has continued to plague the industry.
It is well-known that consistent viscosity is a critical factor in glass suspension application to a sealing surface. It is also well-known the viscosity of a paste may change markedly if the paste is not used promptly after mixing. This tendency becomes serious when long intervals occur between preparation of a suspension and its use. Delays can occur due to interruptions in the sealing process; if an inventory or suspension is desired; or in case of overseas shipments where lengthy transportation and storage times are inevitable.